1. Field of the Invention
This invention relates to a permanently hydrophilic membrane on an aromatic polysulphone basis, and to a method for its production.
2. Description of the Related Art Including Information Disclosed Under 37 CFR .sctn..sctn.1.97-1.99
Synthetic polymer membranes, in particular microporous membranes, have been known for a long time and are generally used for the filtration of liquids and gases, as for example described in "Kesting, R. E.: Synthetic Polymeric Membranes, McGraw Hill Book Co., New York, 1971". Used as polymer materials are for example, polyolefins, polyamides and aromatic polysulphones.
A majority of such membranes is utilized for the filtration of aqueous solutions such as for the manufacture of beverages and pharmaceuticals, or for the preparation of processing waters. It is for this reason that such membranes must advantageously be hydrophilic which means that, in their dry state, they must be spontaneously and completely water wettable so that the aqueous medium can be properly filtered through the membrane.
The requirement that such membranes be completely wettable is especially important in practical applications because, on the one hand, the obligatory testing of the membranes as to their suitability should be performed in situ and on the other hand, because it often seems advisable, particularly in the pharmaceutical and food industries, to sterilize the membranes prior to their use in order to avoid contamination from the membrane during its application. Such a sterilization is usually performed by flushing steam through the membranes of filter modules. While in the beverage industry the usual conditions for this process call for a steam pressure of 1 bar (and a corresponding temperature of 121.degree. C.), sterilization in the pharmaceutical industry takes place at a steam pressure of 2 bars (and a corresponding temperature of 134.degree. C.). Most state-of-the-art membranes are incapable of withstanding the high thermal and chemical stresses this involves because membranes constructed of hydrophilic polymer material are unstable against oxidizing agents per se, and hydrophilated membranes, consisting of hydrophobic raw material per se, will lose their hydrophilic qualities relatively quickly during the steam distillation, especially if tensides or the like are added.
Known from EP 005 536 are membranes constructed of hydrophilic amides and generally showing good water wettability. Their high sensitivity to oxidizing agents such as hypochlorite solution and to strongly alkaline or acid solutions greatly restricts their usefulness in the above described fields of application.
Known from EP 0 108 601 are membranes made of hydrophobic polypropylene which, however, must first be hydrophilated at great cost before they are tested for their suitability and before they are used.
Membranes made of aromatic polysulphone prove to be extraordinarily stable against oxidizing agents But they are normally not hydrophilic However, a number of membranes on a polysulphone basis which are water wettable under normal conditions are also known. The production of such hydrophilated aromatic polysulphone membranes is described in EP 0 082 433. By coagulating the polymer solution in a water bath the membranes are produced by the so-called precipitating bath method and supposedly contain "solidified" polyvinylpyrrolidone, according to the inventor's statement. The thus produced membranes are called hydrophilic, their hydrophilic qualities being supposedly characterized by the membranes' water take-up of greater than 11%.
In EP 0 082 433, the hydrophilic quality of the membrane consisting of hydrophobic polysulphone matrix material is brought about by admixing 15-70% polyvinylpyrrolidone (PVP). After having been produced, such membranes are indeed hydrophilic, but after repeated steam distillations at 2 bars steam pressure (and a corresponding temperature of 134.degree. C.), which is mandatory in the pharmaceutical industry, they lose their water wettability increasingly from distillation to distillation.
A similar process is described in DE-PS 40 00 825. In contrast to DE-PS 31 49 976, only about 0.5% polyvinylpyrrolidone relative to the polymer is typically added. The membrane is formed from the polysulphone solution by the influence of air humidity. In both patents, the cross-linking of PVP is mentioned, the chemical cross-linking being effected in particular by the addition of caustic soda lye, however. But both patents have in common that their membranes, after several water vapor sterilizations at 2 bars pressure, are no longer adequately water wettable. Also, after extraction with boiling alcohol and subsequent drying, the membranes are no longer water wettable.
In EP 02 28 072, membranes are described which consist of a mixture of a water-soluble polymer such as PVP or polyethyleneglycol and a membrane-forming polymer, the latter being characterized in that, in unprocessed form, it takes up 2-4% water. This patent expressly differentiates between the various types of aromatic polysulphones, and it points out that only polymers with 2-4% water take-up, i e. of aromatic polysulphones, only polyethersulphone exclusively, and not polysulphone which takes up less water, are suited for the production of hydrophilic membranes.
Because of their extremely advantageous thermal and chemical resistability, and not least also because of their being unobjectionable with respect to their contacting foodstuffs, membranes on an aromatic sulphone basis represent a most desirable material in processes of the pharmaceutical and food industries. But since they are hydrophobic (in particular polysulphone and polyarylsulphones with less than 2% water take-up capability) they would have to be made hydrophilous before they can be used. The current hydrophilating methods, especially for polysulphones and polyarylsulphones, were insufficient, however, to keep the membrane hydrophilic after repeated steam sterilizations or alcohol extractions required for sterilization. This restricted their applicability. It is decisive for a simple application such as the sterile filtration of aqueous media that the membranes can be tested for their functionability without incurring much expense, which means that it should be possible to test the membranes in their filter holders or filter modules without having to disassemble them from the filtering system. If they are used repeatedly, this is possible only if the membranes remain fully water wettable also after several sterilizations. A membrane filter having just one hydrophobic spot does not pass the test because the test gas will pass through this hydrophobic spot and the membrane must be rejected for safety reasons, although it would still have been intact as such.